1. Field of the Invention
The present invention relates to an actuator composed of a sintered body of thin layer having as its principal component a piezoelectric ceramic layer containing Pb, Zr and Ti, and a manufacturing method of the actuator, as well as a printing head that is suitable for inkjet printer used in printing characters and images.
2. Description of Related Art
In recent years, as a printer outputting information to a printing medium, the use of printers of inkjet type is being rapidly expanded along with the spread of personal computers and the development of multimedia.
A printing head is mounted on such an inkjet printer. As the printing head of this type, thermal head type and piezoelectric type are generally known. In the thermal head type, by a heater disposed in an ink passage filled with ink, the ink is heated and boiled to cause bubbles in the ink passage, so that the ink is pressurized and discharged through an ink outlet orifice. Whereas in the piezoelectric type, the wall of a part of an ink passage filled with ink is bent and displaced by displacement elements, so that the ink in the ink passage is mechanically pressurized and discharged through an ink outlet orifice.
A printing head used for inkjet printer of the piezoelectric type is disclosed, for example, in Japanese Patent Unexamined Publication No. 10-151739. That is, the printing head of this type is, as shown in FIG. 4(a), an actuator 21 is provided via an adhesive layer 21a on a passage member 23 in which a plurality of grooves are arranged in parallel as ink passages 23a, and partition walls 23b are formed as a wall for partitioning the individual ink passages 23a. 
The actuator 21 is provided with a plurality of displacement elements 27 which are obtained by forming a common electrode 25 on one surface of a piezoelectric layer 24 and a plurality of separate electrodes 26 on the other surface. The actuator 21 and passage member 23 are bonded to each other such that the separate electrodes 26 are arranged immediately above the ink passage 23a that is an opening part of the passage member 23.
Then, by applying a driving voltage from a drive circuit to between the common electrode 25 and the separate electrode 26, the displacement elements 27 are vibrated and they constitute part of the ink passage 23a. Thereby, the ink in the ink passage 23a is pressurized and a drop of ink is discharged from an ink outlet orifice 28 that is opened in the bottom surface of the passage member 23.
In this construction, when a driving voltage is impressed to the common electrode 25 and separate electrode 26, although the piezoelectric layer 24 extends in the direction of plane direction dX, a portion (constraint part) of the piezoelectric layer 24 is constrained which is joined to the partition walls 23b disposed in the periphery of the ink passage 23a. Therefore, a portion of the piezoelectric layer 24 which is not constrained (non-constraint part) deflects in the direction of thickness direction dY, and pressure is exerted on the ink passage 23a, so that a drop of ink is discharged through the ink outlet orifice 28 connected in communication to the ink passage 23a. 
As shown in FIG. 4(b), the printing head is formed by disposing in parallel a large number of separate electrodes 26 on the piezoelectric layer 24, and providing a large number of displacement elements 27. Independent control of the individual displacement elements 27 contributes to higher speed and higher accuracy of the inkjet printer.
Japanese Patent Unexamined Publication Nos. 11-34321 and 7-315923 disclose that the above-mentioned actuator can realize a piezoelectric element excellent in thermal resistance, durability and temperature change resistance, by using a perovskite compound containing, as a metal component, at least one selected from Pb, Zr, Ti, Zn, Sb, Ni, Te, Sr and Ba.
In the case of using such an actuator as the printing head of an inkjet printer, it is important to suppress displacement variation in the actuator. However, when a sintered body is used as a piezoelectric ceramic layer, there is the problem that displacement variation is large because of large variations in characteristics. As the result, due to variations in ink discharge, there occur large variations in the density of characteristics and images, so that contrast is poor and high-speed printing is impracticable.
In recent high-accuracy printers, there is a tendency of reducing thickness in order to increase displacement as an actuator. Therefore, in order to directly obtain a piezoelectric sintered body having a thickness of, for example, 100 μm or less, particularly 60 μm or less, when firing is carried out at high temperatures of 1000° C. or more, Pb evaporates from the surface of a molding body, thus causing a composition change and variations in the composition of the obtained piezoelectric sintered body. The use of such an actuator as the printing head of an inkjet printer suffers from the problem that displacement variation is increased to cause variations in ink discharge and a partial deterioration in the contrast of characters and images to be printed, thus failing to perform high-speed printing.
Additionally, in order to obtain an actuator of 100 μm or less, if an attempt were made to use only a central part of less composition variation by removing a surface part from which Pb has been evaporated, from a thick piezoelectric porcelain obtained by firing, it is actually difficult to cut ceramics in a small thickness of 100 μm or less. It is therefore difficult to manufacture an actuator that is composed of a thin ceramics free of composition variation.
In contrast, when the thickness of the actuator 21 is 100 μm or less, there is the problem that the residual stress during firing and shrinkage causes deformation and the stress caused by the restraint of the partition wall 23b reduces the displacement amount of the displacement element 27.
Additionally, since the adhesive layer 21a is not controlled to a specific thickness, when displacement elements 27N corresponding to the adjacent ink passage 23a cause displacement simultaneously, there is the following problem. That is, the constrained portion of the piezoelectric layer 24 that must be constrained by the partition wall 23b does not exert sufficient constraining force, so that the adjacent displacement elements 27 and 27N interfere with each other, thereby increasing displacement variation. This problem becomes significant especially when the partition wall 23b has a small thickness.